1. Field of the Invention
The present invention relates to a radiation imaging apparatus and system. More particularly, the present invention relates to a radiation imaging apparatus and system in which the start and end of irradiation is recognized according to a detected radiation dose, and in which reliability of the recognition of the start and end of the irradiation can be high.
2. Description Related to the Prior Art
A radiation imaging system is well-known medical system for use with X-rays as radiation. The radiation imaging system includes an X-ray generation apparatus and an X-ray imaging apparatus for creating an X-ray image by receiving X-rays from a body (object) of a patient. The X-ray imaging apparatus includes a radiation source, a controllable power supply (source control unit), and a radiation switch. The controllable power supply drives the radiation source. The radiation switch is operable for inputting a signal for starting the irradiation. The X-ray imaging apparatus includes a radiation detector and a console unit. The radiation detector detects the X-rays from the body to create the X-ray image. The console unit controls the radiation detector and also displays the X-ray image.
In a recent type of the radiation detector in the radiation imaging system, an FPD device (flat panel detector device) or detector panel is incorporated instead of an X-ray film or imaging plate (IP). The FPD device includes numerous arrays of pixels for storing signal charge according to dose of X-rays. The signal charge is stored by each of pixels in the FPD device, and converted into a voltage signal by a signal processing unit, to detect the X-ray image of information of an object image. Image data of a digital form of the X-ray image is output.
As a portable type of the radiation detector, an electronic cassette as a radiation detecting cassette is known, and includes a quadrilateral housing and the FPD device contained in the housing. The electronic cassette is removably mounted on a floor stand or frame support for examination in an upright position. The floor stand is a well-known device commonly used with any one of a film cassette, IP cassette and various units. In contrast with an installed type of detector, the electronic cassette is also placed on a bed, or held manually by a patient for imaging a body part which could not be imaged easily with the installed type. Also, the electronic cassette is transported out of a hospital for the purpose of emergency medicine for a patient of an accident or disaster, or diagnosis of a home-care patient.
In the FPD device, resetting is carried out to discharge dark current or unwanted charge of pixels of images of earlier imaging in order to minimize influence of noise to the X-ray image. In the radiation detector with the FPD device in general, it is necessary to synchronize a start of the irradiation with an end of the resetting to start the storing. To this end, a communication interface is disposed between the controllable power supply and the radiation detector. A start signal is generated by the controllable power supply for the irradiation, and transmitted input to the radiation detector. The storing of the radiation detector is triggered by receiving the start signal.
Also, an AEC (or automatic exposure control) is carried out in the radiation detector. A radiation sensor or dosimeter is associated with the radiation detector, and detects a radiation dose of X-rays passed through the body. When an accumulated value of the radiation dose from the radiation sensor increases and becomes equal to a predetermined threshold, the irradiation of the X-ray imaging apparatus is stopped, to change over the radiation detector from the storing to the reading.
It is likely that no communication interface is present with the controllable power supply. In one type of the radiation detector, a start and end of the irradiation are recognized according to a comparison of the radiation dose with a threshold. The radiation detector starts the storing if the start is recognized. The radiation detector changes over from the storing to the reading if an end of the irradiation is recognized.
In U.S. Pat. Nos. 6,516,098 and 6,643,411 (corresponding to JP-A 11-151233), the radiation sensor is disposed behind the FPD device directed to the body. The start and end of the irradiation is recognized according to an output of the radiation sensor. According to the document, a separate structure of the radiation sensor from the FPD device is suggested. Also, disposition of the radiation sensor is not limited to the backside of the FPD device.
U.S. Pat. Nos. 6,999,121 and 7,436,444 (corresponding to JP-A 2002-181942) discloses a type of the X-ray imaging apparatus in which the radiation sensor is disposed inside a housing or chassis for containing the FPD device. JP-A 2011-185622 discloses the X-ray imaging apparatus in which a bias voltage is applied to pixels of the FPD device and the radiation dose is detected by monitoring a current flowing in a bias line for supplying a bias voltage. The bias line and a current meter for measuring the current in the bias line are combined to correspond to the radiation sensor.
Various problems may occur upon delay of the start and end of the irradiation. If the recognition of the start delays, part of the charge according to X-rays applied before the recognition is not stored in pixels. Degradation of the X-ray image occurs. Also, the body of the patient is likely to receive unwanted radiation. It is necessary to recognize the start and end of the irradiation quickly. For example, time of the irradiation for imaging the chest is as short as 50 msec. The start and end of the irradiation must be recognized in a very short time.
To detect the radiation dose of X-rays passed through the body as described in the above documents is influenced by attenuation of the X-rays after the passage. There occurs a problem in the irradiation at a relatively low dose of the radiation dose. The detection of the start and end of the irradiation may be delayed. Also, failure in the detection of the start and end of the irradiation may occur.
In the above documents, the radiation sensor is stationary and does not have freedom in the disposition. Influence of the attenuation to the imaging occurs in the presence of the body. Although it is disclosed in U.S. Pat. Nos. 6,516,098 and 6,643,411 that the disposition of the radiation sensor is not limited, the position of the radiation sensor may be modified as desired by a user for attachment. The radiation sensor cannot be moved form the set position once the radiation sensor is positioned. There is no suggestion of degree of freedom in positioning the radiation sensor.